The principle application area for CIG has been that of visual training simulators which present scenes to an observer or trainee to allow the observer to practice some task, such as flying an airplane. In a flight simulator, a three-dimensional model of the desired "gaming area" is prepared and stored on magnetic disk or similar bulk storage media. This model is called the visual data base. The visual simulator combines an image generator with an electro-optical display system such as a cathode ray tube (CRT) or similar display. The image generator reads in blocks of three-dimensional data from the disk and transforms this data into two-dimensional scene descriptions. A single simulator may be required to provide several different scenes, as, for example, in the case of an airplane having several windows in the cockpit. The two-dimensional data are converted to analog video that is presented to the operator via the display. The generated imagery is meant to be representative of the true scenes that the operator would see if the operator were actually performing the task being simulated. The generation of the display images is said to be in "real time" which is normally taken to mean 30 frames per second, as in the U.S. television standard. CIG systems are described in detail in the book entitled Computer Image Generation edited by Bruce J. Schacter and published by Wiley-Interscience (1983).
Absolute realism is not achieved in CIG systems, but fortunately the training mission can be accomplished satisfactorily despite this apparent drawback. Recent, developments have, moreover, remarkably improved the degree of realism attainable. Among the factors affecting the design of visual systems are texture and shading. Natural scenes are rich in texture, and the human eye relies heavily on texture cues to perceive the structure of a scene. In a flight simulator, suitable texture is required to create the visual sensations of air speed, altitude, horizon angle, and location in the environment. From a scene-processing point of view, the addition of texture to the displayed faces, i.e. planar portions of a modeled surface of the simulated scene, increases the information content without increasing the number of faces to be processed. Recent work in "cell texture" has been especially effective in this respect. This approach is described in detail in application Ser. No. 06/527,809 filed Aug. 30, 1983, by Bunker et al and entitled "Advanced Video Object Generator".
When a face is illuminated by both direct sunlight and ambient light, there are in general three components of the surface illumination; ambient (such that a point has the same brightness at every angle of observation), specular reflection (such that the angle of reflection equals the angle of incidence for a perfect reflector and the amount of reflection reaching the eye if the reflector is imperfect is related to some power of the cosine of the angle between direction of reflection and the line of sight), and diffuse reflection (reflected from a given point equally in all directions but proportional to the cosine of the angle between the light source and the reflecting surface). Visual simulators do not generally display the specular component. It has been shown that the appearance of a diffuse-reflecting curved surface can be given to planar faces by performing a linear interpolation of the intensity values (or "shading values") of the vertices of the polygons that define the planar faces. The hardware required to do this in real time is extensive and, as a result, the effect is used sparingly in actual systems.